The last British resident being held in Guantánamo Bay has been tortured and prevented from seeing his lawyer in a breach of international law, according to documents seen by the Guardian.

Shaker Aamer, who has been held in solitary confinement since January 2005, was assaulted in July by military guards dressed in riot gear who also cut off his clothes using "rough shears" and left him naked and shivering, according to a witness statement from another detainee.

Aamer's lawyers claim the US authorities tried to cover up the alleged attacks which they say took place over a two-day period when Aamer had been due to meet his US lawyer Brent Mickum.His legal team in the UK has asked the foreign secretary, David Miliband, to intervene

Aamer was born in Saudi Arabia, His British wife and four children live in south London and his detention has caused a diplomatic row between the UK and US. Earlier this year Miliband had a request to Hillary Clinton, the US secretary of state, for Aamer to be released into British custody turned down on "security" grounds.

Aamer's lawyers say he has been singled out because he organised a hunger strike and a prisoners' council at the base. Mickum said: "They want to wear him down... the US perceives he is a threat."

Mickum had been due to meet Aamer on 22 July but was told Aamer did not want to see him. However, another prisoner states that the attack on Aamer prevented him from meeting Mickum.

Mickum said Aamer has also been subjected to regular abuse since being transferred to Guantánamo Bay. He said: "The British government needs to demand that Shaker is released. Shaker has been jailed for as long as anyone, undergoing regular torture from beatings to food and sleep deprivation, and there isn't a shred of evidence against him."

A Foreign Office spokesman said senior figures continued to press for Aamer's release. He added: "We contacted the US for clarification in respect of the allegations made and have been informed the matter is being followed up."

Until 1996 Aamer had worked in London as a translator for various lawyers. He was close friends with Moazzam Begg, one of the British citizens released from Guantánamo in January 2005, who now campaigns for Aamer's release.

The pair travelled to Afghanistan with their families in 2001 where they say they were helping to build a school for girls as well as boys as part of their religious duty of zakat - or charity. When the families fled ahead of the US-led invasion both were handed to US forces.

A spokesman for the US defence department declined to comment on the specific chargesrelating to Aamer but said all "credible allegations" of abuse were taken seriously. He added that it would never be acceptable to leave a detainee naked under an air conditioning unit.